1. Field of the Invention
The present invention relates to feed injection systems and, in particular, to feed nozzles used for catalytic cracking processes.
2. Description of Related Art
In a typical Fluid Catalytic Cracking Unit (FCCU) consisting of a regenerator, a riser reactor and a stripper, such as that shown in U.S. Pat. No. 5,562,818 to Hedrick which is incorporated herein by reference, finely divided regenerated catalyst is drawn from the regenerator through the regenerator standpipe and contacts with a hydrocarbon feedstock in a lower portion of a reactor riser. Hydrocarbon feedstock and steam enter the riser through feed nozzles. The mixture of feed, steam and regenerated catalyst, which has a temperature of from about 200xc2x0 C. to about 700xc2x0 C., passes up through the riser reactor, converting the feed into lighter products while a coke layer deposits on the surface of the catalyst. The hydrocarbon vapors and catalyst from the top of the riser are then passed through cyclones to separate spent catalyst from the hydrocarbon vapor product stream. The spent catalyst enters the stripper where steam is introduced to remove hydrocarbon products from the catalyst. The spent catalyst containing coke then passes through a stripper standpipe to enter the regenerator where, in the presence of air and at a temperature of from about 620xc2x0 C. to about 760xc2x0 C., combustion of the coke layer produces regenerated catalyst and flue gas. The flue gas is separated from entrained catalyst in the upper region of the regenerator by cyclones and the regenerated catalyst is returned to the regenerator fluidized bed. The regenerated catalyst is then drawn from the regenerator fluidized bed through the regenerator standpipe and, in repetition of the previously mentioned cycle, contacts the feedstock in the lower riser.
The most critical element of the FCCU riser reactor design is the feed injection system. For peak performance, it is essential that the feed injection system distributes the feed in fine spray having a uniform coverage across the riser and a narrow droplet size distribution. Such a spray increases the surface area of the feed droplets and facilitates intimate contacting with the regenerated catalyst. Existing feed injection systems of prior art, however, have difficulty in achieving this desired performance.
A typical FCCU can have either side entry nozzles or bottom entry nozzles to introduce the hydrocarbon feed into the riser reactor. Bottom entry nozzles introduce the hydrocarbon feed from the bottom of the riser reactor whereas side entry nozzles introduce the feed from the periphery of the riser reactor and at a higher elevation. Most modern CCUs are designed with side entry nozzles, such as that shown in xe2x80x9cEvolutionary Design Changes Mark FCC Processxe2x80x9d by J. R. Murphy which is incorporated herein by reference. For FCCUs with side entry configuration, regenerated catalyst is transported upwards from the bottom of the riser by fluidizing gas, usually steam, and the hydrocarbon feed is injected by multiple nozzles mounted on the periphery of the riser reactor at a higher elevation. Modern side entry nozzles, such as disclosed in U.S. Pat. No. 5,794,857 are, in general, good feed atomizers. However, the side entry configuration has several significant drawbacks. The higher feed injection point leads to lower riser reactor volume and lower catalyst circulation, due to higher pressure drop in the riser. The contact of hot, regenerated catalyst with transport steam at the lower riser also leads to higher catalyst deactivation before feed contacting.
Catalytic cracking units with bottom entry nozzles can avoid the drawbacks of the side entry configuration described above. However, prior art bottom entry nozzles are, in general, not as good in feed atomization. U.S. Pat. No. 4,097,243 disclosed a bottom entry nozzle design with multiple tips to distribute feed into multiple streams. Feed atomization was rather poor. In addition, feed was injected in a substantially longitudinal direction of the riser which leads to slow mixing between the feed and the regenerated catalyst because both are moving in a substantially parallel direction. This leads to an undesirable condition of feed contacting with a broad feed vaporization zone in the reactor riser. A number of improvements, such as CA-A-1,015,004, U.S. Pat. No. 4,808,383, U.S. Pat. No. 5,017,343, U.S. Pat. No. 5,108,583, and EP-A-151,882 disclose various means to improve feed atomization for bottom entry nozzles. However, feed atomization remains inadequate, and the feed injection remains substantially longitudinal, leading to slow mixing with regenerator catalyst and undesirable feed contacting in a broad vaporization zone.
U.S. Pat. No. 4,784,328 and EP-A-147,664 disclose two complicated designs of mixing boxes at the bottom of the FCCU reactor riser to improve mixing between feed and regenerated catalyst. However, these mixing boxes have a very complicated geometry with many passages which make it difficult to retain their mechanical integrity and proper functions over time because the lower riser region is extremely erosive.
U.S. Pat. No. 4,795,547 and U.S. Pat. No. 5,562,818 disclose two bottom entry nozzles with different designs of diverter cones at the exit of a substantially longitudinal feed pipe carrying atomized feed. The function of these diverter cones is to redirect the substantially axially flowing feed stream to a somewhat radially discharging feed at the exit, thus intended for enhancing the mixing with the regenerated catalyst. However, there are major drawbacks in these diverter designs. First, the hydrocarbon feed is atomized upstream of the diverter and when the atomized feed impinges on the surface of the diverter cone at the exit, re-coalescence of many of the atomized feed droplets occurs, leading to the formation of sheets of liquid discharging from the cone. The diverter cone achieves a change in the direction of the feed but this comes at the high price of significantly worsening feed atomization. Second, the radially discharging feed in the form of liquid sheets from the diverter cone can penetrate through catalyst in the riser without much vaporization and impinges on the riser wall, leading to major mechanical damage.
The instant invention provides an improved feed injection system for use in catalytic cracking processes which will result in better feed distribution in the reactor riser. One objective of the instant invention is to improve feed atomization and distribution of the bottom entry injection system, thus eliminating the need for a side entry configuration and its drawbacks. Another objective of the instant invention is to discharge the atomized feed in a substantially radial direction for better mixing with regenerated catalyst, but without using a diverter cone. Yet another objective of the instant invention is to be able to discharge the atomized feed in a substantially radial direction, but not to impinge the riser wall. Yet another objective of the instant invention is to be able to discharge the atomized feed in a substantially radial outward direction with multiple fan sprays, thus achieving a uniform feed distribution across the riser.
In a fluid catalytic cracking unit (FCCU), the feed injection system is one of the most critical designs. For peak performance, the feed injection system must distribute the hydrocarbon feed in a fine spray having a uniform coverage across the riser and a narrow droplet size distribution. The present invention improves feed atomization of bottom entry injection systems, thus eliminating the need for a side entry configuration and its drawbacks. The present invention also provides superior feed distribution, compared to that of prior art side entry nozzles.
The improved feed injection method and apparatus includes at least one nozzle which comprises:
a first conduit for providing a passageway for enabling a first dispersing gas to floe therethrough;
a first cap covering the end of said first conduit, said first cap including at least one outlet passageway therethrough adapted for discharging said first dispersing gas in a generally radial outward and upward direction into a liquid hydrocarbon feed to form a mixture thereof;
a second conduit enclosing said first conduit and said first cap and spaced therefrom for providing a passageway for enabling a liquid hydrocarbon feed material to flow therethrough;
a second cap covering the end of said second conduit, said second cap including at least one segment of a circularly-shaped slot radially offset from the axis of the nozzle and serving as an outlet passageway therethrough, which passageway is substantially aligned with said outlet passageway on said first cap and is adapted for discharging a mixture of said liquid hydrocarbon feed and said first dispersing gas, said second cap being spaced from said first cap thereby forming a mixing zone therebetween for mixing said liquid hydrocarbon feed and said first dispersing gas; and
a third conduit enclosing said second conduit and spaced therefrom for providing a passageway for enabling a second dispersing gas to flow therethrough and into said riser reactor.
In a preferred embodiment said second cap includes a plurality of segments of circularly-shaped slots radially offset from the axis of the nozzle forming outlet passageways therethrough adapted for discharging said mixture of liquid hydrocarbon feed and said first dispersing gas in multiple fan sprays and in a generally radial outward and upward direction.